A step of producing semiconductor devices by cutting a Group III nitride semiconductor wafer having an n-type layer, an active layer and a p-type layer stacked on an insulating substrate such as sapphire into chips, includes a step of forming separation grooves in a chip form by exposing the n-type layer to etching, a step of polishing the substrate to decrease its thickness, a step of exposing the substrate by introducing the diamond blade of a dicing saw into the separation grooves, a step of forming a scribe line along the trace for dicing by using a diamond blade of a scriber, and a step of obtaining chips by pushing and dividing the substrate, as disclosed in, for example, Japanese Patent Application Laid-Open {kokai) No. 5-343742.
Japanese Patent Application Laid-Open (kokai) No. 11-354841 discloses a step of cutting and separating, including a step of forming separation grooves in a chip form by exposing the n-type layer by etching, a step of polishing the substrate to decrease its thickness, a step of exposing the substrate by introducing the diamond blade of a dicing saw into the separation grooves, a step of forming a scribe line from the back surface side of the substrate at a position corresponding to the dicing line by using a scriber, and a step of obtaining chips by pushing and dividing the substrate.
It has been described that the sapphire substrate and the Group III nitride semiconductor layer are hard and cannot be divided into chips by cleavage unlike GaAs and GaP and, hence, the thickness of the substrate must have been decreased, before dividing it into chips, so as to make it more easily divided, and that stress-concentrating portions must be formed for dividing, or that dicing or scribing is necessary to locally decrease the thickness to accomplish the division at desired positions. The diamond blade of the dicing saw is usually in the shape of a disk which is dedicated to linear machining, and is not capable of effecting polygonal machining or curved machining. Even a method of forming a marking-off line on the Group III nitride semiconductor layer or on a sapphire substrate, by a dicing saw with a diamond blade, is substantially linear machining because the work to be machined has a hardness comparable to the hardness of the machining material, and it is difficult to accurately form a marking-off line in the form of a polygonal line or in the form of a curve. Therefore, the chip form of the conventional Group III nitride semiconductor device was of a square form.
In the Group III nitride semiconductor light-emitting device, on the other hand, light emitted from the active layer travels to go out of the Group III nitride semiconductor light-emitting device but cannot go out from the surface of the chip due to the relationship of the refractive index and the light is reflected and is absorbed by the Group III nitride semiconductor, by the sapphire substrate or by the electrode metal, and is converted into heat. The ratio of light going out of the chip is called light extraction efficiency. The light extraction efficiency at the end of the chip is greater when the chip is of a polygonal form than when it is of a square form, and becomes a maximum when the chip is of a circular form. This is because the conditions for perpendicular incidence on the end surfaces from the center of the chip consist of four conditions in the case of a square form, six conditions in the case of a hexagonal form and perpendicular incidence is permitted under every condition of 360 degrees in the case of a circular form. Therefore, the light extraction efficiency on the end surfaces of the chip can be improved in the case of the hexagonal chip as compared to the square chip.
Japanese Patent Application Laid-Open (kokai) No. 9-082587 discloses a method of producing a hexagonal chip by using a conventional machining technology. Referring to FIG. 4 of this patent document, the chips are divided by forming separation grooves which are linear machining lines in a manner that triangles and hexagons neighbor each other. That is, triangular portions are rounded off to obtain hexagonal chips. Japanese Patent Application Laid-Open {kokai) No. 2000-164930 discloses the arrangement of electrodes of the Group III nitride semiconductor light-emitting device of a hexagonal form. However, this patent document is quite silent about the method of producing the hexagonal chip.
In recent years, there has been developed a device for forming separation grooves for cutting chips by using a laser beam as disclosed in, for example, U.S. Pat. No. 6,413,839. The laser beam can be used not only to simply substitute for the conventionally employed dicing saw or the scriber but also is a machining technology that offers unknown probability which may realize a machining method that could not be accomplished by conventional methods. For example, Japanese Patent Application Laid-Open {kokai) No. 10-044139 discloses a technology for cutting by irradiating the bottoms of the separation grooves that have been formed in advance with a laser beam to cause a local thermal expansion. The laser beam is not only the heating means but also is capable of forming separation grooves having any depth or width by controlling the diameter of the beam, position of the focal point thereof, laser output and irradiation time. As an example, Japanese Patent Application Laid-Open {kokai) No. 11-163403 discloses a technology for forming separation grooves in the surface of the side opposite to the surface irradiated with the laser beam.
A polygonal chip has a number of sides greater than that of the conventional square chips and, hence, for example, a light-emitting device features an improved light extraction efficiency at the end surfaces of the chip. According to the conventional method of producing a hexagonal chip as described above, a triangular chip and a hexagonal chip are obtained by linearly forming the machining lines by using a dicing saw or by a scribing method. According to this method, however, the areas of the triangular shape are lost, and the area efficiency becomes poor. The light-emitting device of a polygonal chip can be expected to provide a high brightness. According to the conventional method of machining the polygonal chip, however, a problem remains in regard to a large machining loss and a poor area efficiency.
According to a laser machining method, a Group III nitride semiconductor or sapphire substrate is removed from the semiconductor surface side or the sapphire substrate side of the Group III nitride semiconductor device by sublimation from a portion irradiated with a laser beam which has a beam diameter of an order of microns, and there can be formed separation grooves which are deeper and narrower than that formed by the dicing method within short periods of time. However, if the substrate being machined is warping, the position of focal point of the laser beam undergoes a relative change, and the width and depth of the separation groove vary. A method can be contrived to control the position of the focal point to meet the warping by measuring the warping of the substrate in advance. In many cases, however, the shape of warping varies as the machining by laser proceeds, and a machining precision is not obtained for forming separation grooves of the order of microns in the whole surface of the substrate. To accomplish the laser machining maintaining high precision, therefore, the warping of the sapphire substrate to be machined must be decreased.